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When Bigger Isn't Better

The evolution of bigger bodies and fussier food preferences led to the demise of ancient dogs. Researchers combing the fossil record back to 50 million years have discovered that the switch to an Atkins-like diet of meat, meat, and more meat made these early carnivores susceptible to extinction. The work helps explain a puzzling pattern of extinction in mammals.

Over a hundred years ago, discoveries by paleontologist Edwin Cope convinced him that animals got larger over evolutionary time. Since then, others have shown that this trend holds in several groups of mammals. Today's horses, for example, are 10 times heavier than their counterparts from 50 million years ago. Surprisingly, the fossil record shows over and over again that once species get big, they tend to disappear.

To investigate why that might be, Blaire Van Valkenburgh, a paleobiologist at the University of California, Los Angeles, examined teeth and jawbones from extant and extinct canids to determine how big the animals were and what they ate. Deeper jaws and teeth built for shearing belong to meat-eaters, while omnivores have shallower jaws and grinding molars. The size of the forward-most molar can be used to estimate an animal's size.

Canids consist of modern dogs and their relatives and two extinct groups. The mass of the extinct ones increased up to 600% over about 30 million years, and they became "hypercarnivores" that ate only hunks of meat from large prey, Van Valkenburgh and colleagues report in the 1 October issue of Science. During that time, large members of these groups disappeared. They were replaced by small carnivores, including the line that led to the modern dog, and none of these species switched to hypercarnivory. The researchers determined that the meat-loving species tended to survive just 6 million years, compared to 11 million years for smaller species with more balanced diets. Based on the succession of fossils, the researchers also suggest that once animals specialize for capturing big prey, they can't evolve back to eating smaller prey or vegetation.

The study reinforces the idea that animals that become large and overspecialized are more susceptible to extinction, comments James Hanken of Harvard University, as it seems the larger population sizes of small species enable them to withstand the vagaries of their environment better than large predators that exist in fewer numbers because of the resources they need to support their bulk. The results may be applicable to other animals too. "All the basic premises and circumstances that are invoked or demonstrated here are manifest in many other [groups], beyond both canids and carnivores."